People don't realise stormwater run-off has chemicals in it which need treatment to make them safe, he says.

New Zealand doesn't have the framework to control cross-pipe contamination and there would still be a risk of inadvertent drinking from outside taps, he says.

"It's a real risk. A lot of people say it's a small risk but we would say that any small risk is too big."

An Auckland Council spokesperson says the project was given the green light initially because of the sustainable values it promoted.

The council reviewed the situation in December and decided to continue operating without the recycling system until another review was conducted this year.

"The initiative was based on considerations of environmental sustainability, however the proposed operation and maintenance of the scheme would entail significant cost considerations and potential water quality issues."